The long-term objective of this study is to understand the mode of action of genes controlling notochord development. The notochord plays a pivotal role in patterning axial mesodermal structures and the central nervous system during embryogenesis. Defects in the notochord lead to malformations or agenesis of the axial skeleton and of the spinal cord. Whereas the functions of the notochord are well established, little is known about the genetic elements that control its development and integrity and thus influence pattern forming processes. The existence of mutations with different specific effects on notochord formation suggests that i) the formation of the notochord in the trunk and tail are developmental processes governed by distinct genetic elements, and ii) these genes act specifically at different times and in different regions along the anterior-posterior axis. To understand the molecular mechanisms underlying the apparent temporal and spatial differences of notochord development and integrity, the isolation and functional characterization of genes involved in the regulation of these processes is essential. The analysis of mutants in which the normal development and the integrity of the notochord are specifically perturbed at different times and in different regions along the body axis is the key to isolate such genes. In this proposal we will focus on the analysis at the cellular and molecular level of 3 such mutations: Danforth's short tail (Sd), which affects the notochord early along its entire length, truncate (tc), which acts later and only on the caudal part of the notochord, and Etl4lacZ, a novel lacZ-enhancer-trap-induced mutation at the EtI4 (Enhancer-trap- Iocus-4) gene which affects only the very posterior end of the notochord and is closely linked to but distinct from Sd. The specific aims of this proposal are to: l) study the cell autonomy of the action of these genes and assess the developmental potential of cells carrying these mutations in chimeric embryos 2) analyze the gene mutated by the Etl4lacZ insertion and determine its function 3) clone the Sd gene by chromosomal walking with YAC clones starting at the closely linked Etl4 gene, determine the genetic basis for the Sd mutation and analyze the Sd gene and its function The analysis of these genes and their mode of action should help to understand the biological significance of the apparent regionalization of gene action in notochord development.